PDs are generally used as optical elements in a semiconductor device to convert incident light into an electric signal (current or voltage). Such a PD is typically have either a PN junction structure, an avalanche breakdown (APD) structure or a PIN or NIP structure. PDs with a PIN structure typically include a P type electrode, an intrinsic epitaxial layer, an N+ layer, and a P type substrate. PDs with an NIP structure typically include an N type electrode, an intrinsic epitaxial layer, a P+ layer, and a P type substrate. Currently, PDs are typically manufactured using the NIP or PIN structures.
For example, PDs having an NIP or PIN structure may be used in an optical pickup to record data and/or reproduce data from a CD-ROM, a CD-R/RW, a DVD-ROM, a DVD-R/RW, or the like. PDs may also serve as an interface to transmit a signal to a servo.
FIG. 1 shows a conventional PD having an NIP structure. The PD of FIG. 1 is generally manufactured as follows. A P+ buried layer 2 is formed on a P type substrate 1, and then a P− epitaxial layer 3 is formed on the P+ buried layer 2. First P+ separation diffusion layers 4 are formed. An N type epitaxial layer 7 is formed, and second P+ separation diffusion layers 8 are formed to overlap the first P+ separation diffusion layers 4. Here, ion implantation and thermal diffusion are performed to form the first and second P+ separation diffusion layers 4 and 8. N+ layers 13 are formed to reduce the resistance of the cathodes. A P+ layer 8′ is formed to form split PDs, and cathode contacts 14 and anode contacts 15 are formed.
The performance of a PD may be measured through light efficiency and a frequency characteristic. Research and development are in progress to improve such a performance. However, limiting factors of ion implantation and thermal diffusion performed in manufacturing the PD may make it difficult to further improve the performance of the PD.
As an example, minimizing series resistance of the PD may improve the performance and frequency characteristic of the PD. In FIG. 1, series resistance between the P+ buried layer 2 and the first and second P+ separation diffusion layers 4 and 8 may be the dominant components of the series resistance of the PD. Conventionally, a deep or shallow junction is formed to reduce such series resistance. However, reductions in the series resistance using ion implantation and thermal diffusion may have reached their limit and thus the ability to improve performance by reducing series resistance may be limited in devices using ion implantation and thermal diffusion.
For example, a thermal diffusion process must typically be sufficient to diffuse a high dose of impurities into a lower portion of a PD. However, such a thermal diffusion process may also result in the area of the PD increasing in a horizontal direction. Also, it may be difficult to implant a high dose of ions at a high energy to provide the high dose of impurities in the lower portion of the PD. Thus, horizontal diffusion of the ions may be unavoidable. As a result, the area of the PD may increase.